As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices.
Depending upon kinds of external devices in which the secondary batteries are used, the secondary batteries may be used in the form of a single battery or in the form of a battery pack having a plurality of unit cells electrically connected to one another. For example, small-sized devices, such as mobile phones, can be operated for a predetermined period of time with the power and the capacity of one battery. On the other hand, a secondary battery pack needs to be used in middle- or large-sized devices, such as laptop computers, portable digital versatile disc (DVD) players, small-sized personal computers (PCs), electric vehicles, and hybrid electric vehicles, because high power and large capacity are necessary for the middle- or large-sized devices.
The battery pack is manufactured by connecting a protection circuit to a core pack having a plurality of unit cells (secondary batteries) connected in series and/or in parallel to one another. When prismatic batteries or pouch-shaped batteries are used as the unit cells, the prismatic batteries or the pouch-shaped batteries are stacked such that large-sized surfaces of the prismatic batteries or the pouch-shaped batteries face each other, and then electrode terminals of the prismatic batteries or the pouch-shaped batteries are connected to one another by connection members, such as bus bars. Consequently, when a three-dimensional secondary battery pack having a hexahedral structure is to be manufactured, the prismatic secondary batteries or the pouch-shaped secondary batteries are preferably used as unit cells of the secondary battery pack.
On the other hand, cylindrical secondary batteries generally have electric capacities larger than the prismatic secondary batteries or the pouch-shaped secondary batteries. However, it is difficult to arrange the cylindrical batteries in a stacked structure due to the external shape of the cylindrical secondary batteries. When the secondary battery pack is constructed generally in a line-type structure or in a plane-type structure, though, the cylindrical secondary batteries are structurally more advantageous than the prismatic secondary batteries or the pouch-shaped secondary batteries.
Consequently, a battery pack having a plurality of cylindrical secondary batteries connected in series to or in parallel and series to one another is widely used in laptop computers, portable DVD players, and portable PCs. The secondary battery pack may be constructed in various core pack structures. For example, the core pack of the battery pack may be generally constructed in a 2P(parallel)-3S(series) line-type structure, a 2P-3S plane-type structure, a 2P-4S line-type structure, a 2P-4S plane-type structure, a 1P-3S line-type structure, or a 1P-3S plane-type structure.
The parallel connection structure is achieved by adjacently arranging two or more cylindrical secondary batteries in the lateral direction thereof, while electrode terminals of the cylindrical batteries are oriented in the same direction, and connecting the electrode terminals of the cylindrical batteries to one another using connection members by welding. The cylindrical secondary batteries connected in parallel to one other may be referred to as a “bank.”
The series connection structure is accomplished by arranging two or more cylindrical secondary batteries in the longitudinal direction thereof such that electrode terminals of the cylindrical batteries having opposite polarities are successively disposed one after another, or adjacently arranging two or more cylindrical batteries in the lateral direction thereof, while electrode terminals of the cylindrical batteries are oriented in opposite directions, and connecting the electrode terminals of the cylindrical secondary batteries to one another using connection members by welding.
The electrical connection between the cylindrical secondary batteries is generally achieved by spot welding using thin connection members, such as metal plates (for example, nickel plates).
FIG. 1 typically illustrates a battery pack constructed in a 2P-3S plane-type structure in which batteries are electrically connected to one another by spot welding. For easy understanding, the coupling between the batteries constituting the battery pack of the 2P-3S plane-type structure is shown in an exploded view.
As shown in FIG. 1, three pairs of secondary batteries 20 and 21, connected in parallel to each other for each pair, are connected in series to one another via metal plates 30 to constitute a core pack 10.
FIG. 2 is a typical view illustrating a battery module 50 in which a protection circuit module is connected to the core pack of FIG. 1.
As shown in FIG. 2, secondary batteries 20 and 21 are connected to the protection circuit module 90 via a cathode conducting wire 60 and an anode conducting wire 70 connected to the metal plates 30 and flexible printed circuit boards (FPCB) 80 connected to the conducting wires. The electrical connection between the metal plates 30 and the protection circuit module 90 is mostly achieved by soldering.
Generally, a battery pack using secondary batteries as unit cells is repeatedly charged and discharged during the use of the battery pack, and the battery pack uses lithium secondary battery, which exhibits low safety in abnormal conditions, such as external impact, dropping, penetration of a needle-shaped body, overcharge, overcurrent, etc., as a unit cell. In order to solve such a safety-related problem, therefore, a safety element, such as a protection circuit module, is included in the battery pack. The safety element acquires information, such as voltage, at a corresponding terminal connection region of the battery pack to perform a predetermined safety process, thereby securing the safety of the battery pack. Consequently, when the connection state of the corresponding region is variable, for example, the resistance value of the terminal connection region changes due to vibration, the detected information is inaccurate, and therefore, the safety element cannot perform the desired process. For this reason, the electrical connection between the battery cells and the protection circuit in the battery pack is generally achieved by soldering.
Also, it is necessary to connect a plurality of battery cells in series or in parallel to one another to constitute a high-power, large-capacity battery pack. In addition, a stable coupling method that is capable of minimizing the resistance change of the terminal connection region is required to uniformly maintain the efficiency of the battery pack. Generally, the electrical connection between the battery cells is achieved by soldering or welding, preferably spot welding.
However, the welding or soldering process between the battery cells has the following problems.
Specifically, the welding or soldering process requires worker's skilled technique and know-how. In addition, the control of parameters necessary to decide the intensity of welding must be continuously performed. As a result, the production process is complicated, and the production costs increase, whereby the production efficiency lowers. Also, a short circuit may occur at the welded region, due to the vibration generated from the battery pack or external impact applied to the battery pack, at the time of directly welding or soldering the battery cells. In addition, electrical or thermal damage may be caused between the battery cells and the connection members, whereby the safety of the batteries is threatened, and the defective product rate increases. Furthermore, when some of the battery cells become defective, during the manufacturing or use of the battery cells, all the battery cells constituting the battery pack must be discarded.
Consequently, there is a high necessity for a technology that is capable of substituting for the connection method based on such welding or soldering, which threatens the safety of the batteries and requires a complicated working process, and, at the same time, reusing the remaining battery cells, although some of the battery cells are defective, while stably securing the connection structure between the battery cells.
Meanwhile, for a battery pack using primary batteries, various attempts have been made to achieve the electrical connection between the respective batteries. For example, Korean Patent No. 0413381 discloses a technology for forming conductive coils at opposite ends of battery cases to electrically connect batteries to one another. U.S. Pat. No. 525,037 discloses a technology for mounting metal plates, which are bent to exhibit elasticity, at opposite ends of batteries to achieve electrical connection between the respective batteries.
However, the above-mentioned technologies have a problem in that it is required for connection members to exhibit elasticity enough to fix the battery cells and stably connect electrode terminals to one another, and therefore, connection members exhibiting low elasticity are limited in use. Especially, the technology using the conduction coils has problems in that the sectional area of a wire constituting each coil is small, and the connection length of the wire is relatively large, whereby the electrical resistance increases. The increase of the electrical resistance causes power loss and increases the amount of heat generated, whereby the stable connection between the batteries may be obstructed. Also, for the technology using the metal plates that are bent to have elasticity, the metal plates may lose their elasticity or break when an excessive force is applied to the metal plates at the time of inserting the battery cells into the pack case, or when the metal plates are repeatedly used, with the result that, when external impact is applied to the battery cells, the battery cells may be separated from the pack case or the electrical connection between the battery cells may be cut off.
Furthermore, the above-mentioned connection member is limited to apply to the previously described secondary battery pack due to the variable connection state at the corresponding region.
Also, in order to achieve the electric connection between the battery cells in a mechanical contact manner, without using welding or soldering, it is required that partitions necessary to mount the connection members to the pack case be located between the battery cells, as in the conventional arts. However, the provision of the partitions increases the size of the battery pack, which is far from the latest tendency to pursue the reduction in size, weight, and thickness. In addition, it is preferred for a battery pack including a plurality of battery cells to be under a uniform operating condition in the aspect of the operational efficiency. However, the operating conditions of the battery cells mounted in the receiving parts divided by the partitions may be different from each other for the respective receiving parts, when external impact is applied to the battery pack, through the provision of the partitions.
In this aspect, a method may be considered of mounting mechanical contact type connection members between the battery cells at a very high elastic pressing force in a structure having no partitions. In this method, however, a material, such as polymer resin, for the pack case is slowly deformed by stress during the use of the pack case for a long period of time, which is called a creep phenomenon. Consequently, excessively high elastic pressing force of the connection members causes the occurrence of stress at the pack case, which leads to the creep phenomenon. As a result, the distance between the battery cells gradually increases, and therefore, the electrical connection between the battery cells is unstable. This phenomenon may be serious especially for a device of which the long-term use is required. Consequently, the connection method based on the primary batteries cannot be applied to a battery pack, based on secondary batteries, of which the long-term use is required through repeated charge and discharge, without any modification.
Meanwhile, a cylindrical battery is constructed in a structure in which a jelly-roll is mounted in a metal container, and a protruding cathode terminal is formed at one end of the container while a flat anode terminal is formed at the other end of the container. Since a cap assembly is mounted to the top of the jelly-roll in a crimping structure, the cathode terminal region exhibits structural stability against an external force. On the other hand, since the jelly-roll directly faces the inner bottom of the container, the anode terminal (i.e., the bottom of the container) is deformed by an external force, with the result that a short circuit may occur between electrode plates of the jelly-roll.
In a battery pack including a plurality of battery cells, such a short circuit causes a very serious problem in the aspect of the safety. The inventors of the present invention have experimentally confirmed that such a short circuit occurred in a structure in which connection members, such as nickel plates, are coupled to the electrode terminals of the battery cells by welding.
Consequently, there is a high necessity to provide a connection member for secondary batteries that is capable of substituting for the connection method based on welding or soldering, and securing the stable connection structure between the battery cells and the safety of the batteries while not causing the increase in size of the battery pack.